Production of high titer biofuels from microalgal biomass through energy-saving integrated process

Abstract

Biofuels produced from biomass feedstocks are eco-friendly fuels that can replace fossil fuels and are considered an alternative solution to energy crisis and global warming. First- and second-generation biofuels cause food ethical issues, higher prices, natural and political problems with expensive production processes for large-scale biorefinery production. Microalgae, the third-generation biomass for biofuels production, have high productivity and can be converted into energy carriers, but processes for industrial scale-up is not economical. The most important challenges for eco-friendly, economical biofuels production from microalgal biomass are the technological and engineering parts i.e., energy-efficient pretreatment, and fermentation processes. The energy-saving integrated process can achieve maximum biomass utilization and conversion efficiency from microalgal biomass through the maximum utilization of microalgal biomass compositions and minimum waste generation for sustainable high titer biofuels production. The commercial feasibility regarding conversion of the high-solid-loading microalgal suspensions for high-titer biofuels production is questionable owing to incomplete utilization of biomass and higher processing costs. The physiological properties including biochemical composition and cell wall integrity and thickness of microalgal species have a remarkable effect on the pretreatment and fermentation processes, and their further conversion to biofuels. In this research work, multiple biofuels (bioethanol, higher alcohols (C3-C5), and biodiesel) were produced by energy-efficient microwave pretreatment, fermentation of carbohydrates and proteins (successive fermentation), and transesterification of lipids from three different microalgal strains (Pseudochlorella sp., Chlamydomonas mexicana, and Chlamydomonas pitschmannii), each possessing different proportions of bioconstituents (carbohydrates, proteins, and lipids). The production of multiple highly concentrated biofuels (bioethanol, higher alcohols, and biodiesel) was achieved through cost-effective integrated approach (pretreatment, serial fermentation of carbohydrate/protein, and transesterification of lipid) from highly concentrated (100 g/L) microalgal suspensions (Chlamydomonas mexicana and Chlamydomonas pitschmannii). The cell wall was highly disrupted by bio-pretreatment with yeast cells that reflect the co-extraction of biomolecules during carbohydrate/protein fermentation enhancing extraction efficiency and bioavailability. This integrated approach obtained unprecedented total conversion efficiency (48–63%) and maximum total biomass utilization (77–86%) for all microalgal strains with high yields of bioethanol (0.48 g-ethanol/g-carbohydrate), higher alcohols (0.44 g-higher alcohols/g-protein), and biodiesel (0.82–0.89 g-biodiesel/g-lipid) at suspensions of 100 g/L. The microwave pretreatment method required the lowest specific energy (0.68 MJ/kg) compared to ultrasound pretreatment. Transmission and scanning electron microscopy were employed to visualize the changes in the intercellular morphologies, cell wall structure and thickness before and after serial fermentations. This research suggests that different physiological properties, including cell wall thickness and the proportion of bioconstituents from highly concentrated microalgal suspensions, could have a significant impact on the pretreatment and fermentation efficiencies for biofuels production. Energy-efficient microwave pretreatment enhanced extraction efficiency (carbohydrates, proteins, and lipids) from microalgal strains with different physiological properties and converted to multiple biofuels. This study demonstrated a cost-effective and energy-saving integrated approach to the holistic conversion of high-solid-loading microalgal biomass for the high titer biofuels production with minimum waste generation.